After saving a group of cosmonauts whose spaceship is jeopardized by a rogue satellite, Superman visits his home-town of Smallville disguised as Clark Kent, and checks in on the now uninhabited farm where he grew up. In an empty barn, he uncovers the capsule that brought him to Earth, and removes a luminescent green Kryptonian energy module. A recording left by his mother Lara (voiced by Susannah York) states that its power can only be used once. Unwilling to sell the farm to a mall developer, Superman returns to Metropolis.

Returning to the Daily Planet as Clark Kent, he learns that the newspaper has been taken over by David Warfield (Sam Wanamaker), a tabloid tycoon who fires Perry White and hires his own daughter Lacy (Mariel Hemingway) as the new editor. Lacy takes a liking to Clark and tries to seduce him. Clark agrees to go on a date with her. Following the news that the United States and the Soviet Union may engage in nuclear war, Clark is conflicted about how much Superman should intervene. After receiving a letter from a concerned school-boy, Superman travels to the Fortress of Solitude to seek advice from the spirits of his Kryptonian ancestors. They recommend that he should leave Earth and find a new home. After asking for advice from Lois Lane (Margot Kidder), Superman attends a meeting of the United Nations, announcing to the assembly that he will rid the planet of nuclear weapons. Superman collects most of Earth's nuclear stock-pile in a gigantic net in the planet's orbit, then hurls it into the sun.

Meanwhile, young Lenny Luthor (Jon Cryer) breaks his uncle Lex Luthor (Gene Hackman) out of prison. Returning to Metropolis, the pair steal a strand of Superman's hair from a museum, and create a genetic matrix which Lex attaches to a U.S. nuclear missile. After the missile is test launched, Superman intercepts it and throws it into the sun. A glowing ball of energy is discharged, which develops into a superhuman (played by Mark Pillow and voiced by Gene Hackman). This "Nuclear Man" makes his way back to Earth to find his 'father' Luthor, who establishes that while his creation is powerful, he will deactivate without Solar light. A vicious battle ensues between Luthor's creation and Superman. While saving the Statue of Liberty from falling onto New York, Superman is infected with radiation sickness by a scratch from Nuclear Man's radioactive claws. Nuclear Man kicks Superman into the distance with such strength that his cape is torn away.

To Lois' disgust, the Daily Planet (which has been reformatted as a tabloid newspaper) publishes the headline "Superman Dead?". Lois indicates a desire to quit and seizes Superman's recovered cape for herself. Lacy is also upset and reveals to Lois that she cares for Clark. Lois ventures to Clark's apartment where she proclaims her love for Superman. Felled by radiation sickness, Clark staggers to his terrace where he retrieves the Kryptonian energy module and attempts to heal himself. Having developed a crush on Lacy, Nuclear Man threatens mayhem if they are not introduced. The newly restored Superman agrees to take him to her to prevent anyone else from being hurt. Superman lures Nuclear Man into an elevator car, trapping him inside and then depositing it on the dark side of the moon. As the sun rises, Nuclear Man breaks free due to a crack in the elevator doors and Superman is again forced to defend himself. At the end of the battle, it appears as though Superman has been defeated, and he is driven into the moon's surface by his opponent.

Nuclear Man forces his way into the Daily Planet and abducts Lacy. Superman frees himself from the moon's surface and pushes it out of its orbit, casting Earth into an eclipse which nullifies Nuclear Man's powers. Superman rescues Lacy, then recovers Nuclear Man and deposits him into the core of a nuclear power plant, destroying him. Perry White secures a loan to buy a controlling interest in the newspaper, making David Warfield a minority share-holder and protecting the paper from any further take-overs. In a press conference, Superman declares only partial victory in his campaign, saying, "There will be peace when the people of the world want it so badly that their governments will have no choice but to give it to them". Superman also recovers the fleeing Luthors. Superman places Lenny in "Boys Town" telling the owner that Lenny has been under a bad influence while Lex Luthor is returned to prison. Superman then flies into the dawn for further adventures.
(Source: Wikipedia)